Night do? What does it mean?
Do means something along the lines of party in this instance. A post-wedding party, on the evening of the wedding, probably.
OED has:
DO, n.1
b. Something done in a set or formal manner; a performance; esp. an entertainment or show; a party; hence (orig. jocular), a military engagement, raid, or other ‘show’. Orig. dial. or vulgar.
and some examples of its use first attested to from 1824:
a 1824 J. BRIGGSRemains (1825) 243 Such individuals should have their feast (or do, as it is called).
1925 FRASER & GIBBONS Soldier & Sailor Words 78 Do, an event. A stunt. An attack, etc. E.g., ‘When is the do coming off?’; ‘The Somme do’; ‘The Havrincourt do’, etc.
1955 Times 18 May 14/2 Miss Margaret Herbison broadcast on behalf of the Labour Party last night a talk which she described as a ‘family do’.
1958 M. KERR People of Ship St. ix. 108 Her family has a ‘do’ every year on the anniversary of the day her mother's father died. Ibid., Christmas ‘dos’ are especially important.
Some people in Britain have a fairly small 'do' or reception after the wedding to which close friends and family are invited.
Then they have an evening (or night) 'do', to which many more people are invited.
Traditionally the bride's father paid for the cost of the wedding and the reception. Today, many couples expect to pay for at least part of it themselves. The guests do not expect to pay for their food and drink at the reception.
The evening celebration may well be held in a place with a bar. Guest will expect to pay for at least some of the drinks themselves, though the bride and groom may provide free drinks up to a certain amount/time. Depending on the venue, food may or may not be provided/available.
Some people, of course, still have a large reception with dozens or even, for the super-rich, hundreds of guests. Such people would not know the meaning of the expression 'evening/night do'.